Newby ebike build - Should my 500W system be pulling 1.1KW?

[badger]

Dear All,
Just joined this forum having completed phase 1 of my ebike build. Here's the overview:

- Specialised mountain bike (wife's old one) with front suspension. Wasn't getting used so made an ideal donor bike
- 500W 36V front hub motor and controller. Cheap (£175) kit from ebay. No brand - chinese generic. Controller seems to have 4x Y1707 80A FETs per phase so quite well specced (if you believe the datasheets)
- 10S (actually 6 x 5s 3000 Zippy LiPos in 3p10s config) custom lithium battery pack. 38V nominal 9Ah.
- Road tyres instead of knobbly tread

I've put it all together (and take a lot of care with my battery - balance charge always, don't over discharge beyond 33.5V etc.) and I am absolutely delighted with it. The battery and controller kit probably cost around £300 and with bits and pieces, I think the whole cost is less than £350, and I have had that much worth of fun already just building it.

There's more work to do, like better battery mounting, but it works amazingly well and is used on a daily basis. It takes me to work up some vey steep hills and I drop my daughter off at nursery. She loves it too!

ANyhow - My question. The controller and motor are rated at 500W, but under acceleration, it draws over 1KW (actually peaking around 1100W). Nothing gets that hot (motor is barely warm, controller warm but not hot and batteries cool) and now I have uprated the rather poor 1mm2 wiring, the cables are not burning heat either. I measured it at about 28Amps, peaking at 30, and the voltage droop on the LiPo is minimal. I used a swatt meter which I calibrated at work (I am an electronics engineer).

Is this normal? Would a 500W kit actually draw 500W, or would it be 500W at the wheel, with inefficiencies stacking up to make it draw 1.1KW off the wire.. Is it possible they shipped me the wrong controller. The hub motor looks the same as the 1KW advertised kits and the rating of the controller was scribbled on with pencil...

BTW, I'm not complaining - it goes very well indeed - in fact I need to beware of wheelspin at takeoff! I'm satisfied with range too.

However,If someone can explain the 2x power discrpancy, I'd be much obliged,

Badger

 

[dogman dan]

It's "normal" for a controller with 25 or 30 amps limit, at 36v.

Kinda big for a small controller to that that big an amps limit. It's not a 24v controller by any chance?

I have pretty much given up paying any attention to any watt specifications. The motor can likely stand 1000w easily, provided it doesn't have to do that level for miles and miles, in hot weather.

Unless you are cruising at 30 mph, you will use less than 1000w. 400w can get you 20 mph in good conditions.

 

[badger]

Thanks for the reply dogman.

I'm pretty sure it's a 36V controller since it has the three (green/orange/red) LEDs on the throttle control and the green disappears at 36V exactly. The controller itself backs right off at 33.5V so it wouldn't drive at all on a 24V system.

Interesting to hear that you think that's a big number for a 500W claimed controller. I thought that, hence the post. Again, not complaining!

I only go full power for a minute or two at a time up steep hills so I'm not worried about high duty cycles. On the flat it very quickly reaches it's max speed (about 20mph) and backs off on the current to maintain that speed. I'm happy with that as I don't think the bike is right for higher speeds, without brake upgades etc..

I was considering the shunt mod but 28Amps seems gets me up every hill where I live..

Does anyone know what a typical efficiency might be for the controller/hub motor combo is? I wonder how many of those 1000W actually propel me..

 

[d8veh]

The 500w is the continuous rating, i.e. average. It will run at that all day without problems. Your maximum is 1100w and your minimum is 0w, so your average is probably 500w or less. The main limit in the rating is for overheating, which might be a problem in Arizona, but in the cold UK weather, you could increase that rating by 50%.

P.s. I hope you have at least one torque arm on that motor!

 

[Drunkskunk]

As others pointed out, that 500 watts is the contionus rating. Also caled the Nominal rating. In othwer words, that motor can run at 500 watts output day and night under normal conditions with no problems.
If it happens to be one of the motors we call 9C clones (side covers are 230mm, over all width with spoke flange is ~244mm), then the motor should be good for 1000 watts continous for longer than your battery will last, and peak outputs well over 2000 watts for a few moments.

As a side note, Most reputable vendors rate their motors this way, but many ebay vendors rate their motors by their peak output. you may see that same motor with claims that it is a 1000w or even 1500 watt motor.

The Efficancy for most hub motors is around 80-85% at steady speed, and drops to less when climbing and accelerating

 

[badger]

Wow, thanks for the knowledge guys. So it sounds like, in this case and unusually, a chinese product is quite conservatively rated. I need to get the tape measure out to confirm if it is a "9C clone" but in the mean time, here's a piccy of it. Is this the 9C clone?

Like I said before, the 500, 800 and 1KW offerings all look the same (from the outside at least). I see ebay guys selling a product that looks identical as a 1KW kit. Also, the pricing is pretty close for the 500W vs 1KW - around 15% difference. That sounds like a marketing/positioning deal to me, which is good news for those buying at the low end of the product range.

Maybe I will look at the shunt mod after all... By custom LiPo pack claims it's good for 180A.

As a side thought, armed with my watt meter, a GPS and a big hill, I may do some total system efficiency testing to see how many of my battery joules get turned into m*g*h potential energy.. Could be an interesting experiment and I think I'd be happy if I get 50% after all losses taken into account, given the 80-85% number at steady speed..

 

[d8veh]

Once you go below 50% of maximum no-load speed at full throttle, like when climbing hills, the efficiency starts to drop off rapidly, by the time you get to 25% max speed, it can be as low as 30%. The relationship is a curve, so trying to use that method of determining efficiency is pretty meaningless other than an academic exercise.

 

[badger]

Thanks for the info - I suppose that makes sense if you take the extreme case which is applying torque but not moving. You'll be burning power (probably current limiting) but doing no useful work.

Does that yield a conclusion that, for maximum range, you need to be not going too slowly!? At least compared with the maximum design speed.

By the way d8veh, I didn't know what a torque arm was, but googled it so do now. The answer is I don't have one, but the spindle on the motor is chamfered, and I had to give it a fair old bash to get it into the front forks, so I'm hoping that's enough to deal with the torque.. No issues yet anyhow after a couple of weeks of regular use.

 

[dogman dan]

It's a bit hard to understand at first, but this may make it easier to get a hang on it.

The higher the load, the lower the efficiency. So if you are leaving the stop sign, the highest load is the first foot, and then it reduces load and gains efficiency as rpm increases. If you climb a too steep hill and slow excessively for long enough, you get so inefficient that melting the motor is possible.

BUT, the faster you go, the more power it takes. So 30mph takes more watts, even if it's at 90% efficiency. For example, 30 mph typically takes about 1000w, so at 90% you are getting 900w to the wheel. Sounds good, and it is.

But if you are wanting to go farther on the same size battery, then you must slow down. At 20 mph, you use only 4-500w, putting perhaps 450w to the ground. Half the power, but not half as slow, so in the end you go farther.

Once you start looking at it in terms of watthours per mile, then you see total efficiency rather than just motor efficiency. Wh/mi depends a great deal on how you ride. I can ride my humongous cargo bike with 12 wh/mi, or 40 wh/mi depending on top speed.

 

[wesnewell]

Like I said before, the 500, 800 and 1KW offerings all look the same (from the outside at least). I see ebay guys selling a product that looks identical as a 1KW kit.

The outside doesn't really mean much. It's the inside that counts. My 1000W motor is identical to your 500W motor, but it's not the same inside. You could have 10 motors that look the same but be different inside. The more powerful motor will weigh a little more normally. If the weights are the same, be wary.

 

[d8veh]

If your forks are aluminium or magnesium, you must use a torque arm with that motor. It doesn't matter how tight it is. In fact tightness can make it worse. The problem is metal fatigue, which those metals suffer from. A crack can start some time in the future, weakens the drop-outs, and then without warning they pop. You''ll go straight over the handlebars and land on your face. A torque arm will hold the wheel in place even if the drop-outs pop, and also takes a lot of the stress off the aluminium to reduce the chance of fatigue. Steel doesn't suffer from fatigue, so no problem if the forks are steel.

 

[dogman dan]

You do have to do it right, to use front motors on alloy forks. Generally it starts with a C washer. The number one reason forks fail is the mis fitting oversize washer cracks the dropout when it's tightened. Not overtightend, but just tightened with a poor fit. Then when you ride off, the forks are already cracked.

 

[badger]

ahhh.. well I'm pretty sure they are some sort of light alloy since they were easily filed down. Sounds like I'm taking a risk right now then.. Thanks for the warning! I"m glad I joined this forum... Like I said, this was the first I'd even heard of such a thing. I guess it's not something that you get in the cheap kits.

Would something like this do it?

http://www.ebay.co.uk/itm/Torque-Arm-Plate-Sets-for-Electric-Bikes-/261307062840?pt=UK_SportsLeisure_Cycling_Bike_Lights&hash=item3cd71d5a38

 

[Gregor]

Be very careful if you have an alloy fork, especially if you are towing a child behind. A tight fitting washer on the "shoulder" of the axle that bears against the inside surface of the fork (both sides) and the c-washers that dogman mentioned so that the nuts have a level surface to tighten against. Also avoid controllers that employ regenerative braking as the back and forth torque will mess up that alloy quickly--one well mounted torque arm can give you a lot of peace of mind. Refining and maintaining that sweet ride should give you years of enjoyment. Don't worry about the 1000 watts, soon you will be brave enough to modify that shunt to get 30 amps out of it!

 

[badger]

Thanks everyone for the advice. I have ordered that torque arm from cyclezee (link in previous post) and fitted it. Thought people might like to see a piccy.

Next step is to have a look at the shunt mod.. It's limiting at about 28 AMPs currently so I may set it for about 35 AMPs
- about 25% more. I don't want to go crazy on this. Also I have a 3p2s LiPo to try putting in series to test the effect of 12S instead of 10S (current config).

Great fun this DIY ebike business!

 

[d8veh]

It looks like you've got steel forks, so not as bad as I thought, but I can see that your axle isn't central, which might mean that the first washer isn't sitting in the dimple properly. If so, file the washer until it fits otherwise it can bend and loosen the tension on the nut, which would be bad because torque resistance would be compromised, even with the torque arm.

 

[badger]

Hi d8veh,
you're right that it's not perfect yet. I had a closer look and flange size of the nut and washer is bigger than the shoulder on cup on the fork dropout - hence it's not sitting nicely against flat metal in the recess. This isn't great so I am in the process of adding washers (needed 15mm inside, 22mm outside - bit of a weird size) to make sure everything is seated properly. Luckily there is plenty of thread on the shaft of the hub motor for all of this to fit on. Hopefully, I'm there now..

Thanks for your (and everyone else's) feedback. Really appreciated and hopefully I have now avoided the common pitfalls, and a nasty case of road rash.

I can see how doing a DIY e-bike can be a bit of a minefield for noobs, especially if not mechanically/electrically minded. This is compounded by available and relatively cheap high wattage motors and RC un-managed LiPos. I saw one member (can't recall who) who had a few points on this on his signature. It feels like there is a case for a short and sweet checklist/sticky for noobs, of which there will be more and more as ebikeing grows in popularity, which I'm sure it will given the massive positive experience of ebiking brings!

 

[brumbrum]

These hose clips are much stronger than the one on your torque arm.
http://www.ebay.co.uk/itm/290681277...eName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

If you end up going for more amps all you will get is better pull/speed up very steep hills and chances of popping wheelies when taking off especially when you do a hill start with full throttle. You will also eat up much more juice from the battery and add more strain to your forks. Try putting a magnet to your forks to se if they are steel. But definately get your washers and nuts flush with the fork.

Ebike x man cave = shed engineering....... A way of life.